How to write text file in Android using Action script 3

I tried but it work in PC not in phone.  I can not make work. So I conclude that the phone might close file accessing. I wil contact phone maker.  Thank you.

-----------program--------------

import flash.filesystem.*;
import flash.filesystem.FileStream;
import flash.events.Event;

txtFld.text = "Start";
btnSaveFile.addEventListener(MouseEvent.CLICK,handlerBtnSaveFile);
function handlerBtnSaveFile(e:Event){
txtFld.text = "Pressed";
//var f:File = new File();
//var f:File = File.desktopDirectory;
//f = f.resolvePath("asdf2.txt");
var f:File = File.desktopDirectory.resolvePath("Test.txt");
//f.nativePath = "/mnt/sdcard/asdf.txt";
 

var stream:FileStream = new FileStream();
stream.open(f, FileMode.WRITE);
txtFld.text = f.nativePath;
stream.writeUTFBytes("This is my text file.");
stream.close();
     
}

Dear

I tried reading process. The result is good. First, I copy asdf.txt file into sdcard  then tried program. Here is the code.

--------------program--------------

import flash.filesystem.*;
import flash.filesystem.FileStream;
import flash.events.Event;

txtFld.text = "Start";
btnSaveFile.addEventListener(MouseEvent.CLICK,handlerBtnSaveFile);
function handlerBtnSaveFile(e:Event){
  var f:File = new File();
  f.nativePath = "/mnt/sdcard/asdf.txt";
  //f.nativePath.replace(/\//g, File.separator);
 
  var stream:FileStream = new FileStream();
  //stream.open(f, FileMode.WRITE);
  stream.open(f, FileMode.READ);
  var strFileData:String = stream.readUTFBytes(stream.bytesAvailable);
  //txtFld.text = f.nativePath;
  txtFld.text = strFileData;
 
  //stream.writeUTFBytes("This is my text file.");
  stream.close();
   
}